1. Field of the Invention
The present invention relates to an optical sensor comprising an amorphous silicon layer with a greater light absorption coefficient for visible light to produce photo carriers for use as a photo current to be transmitted via highly mobile polycrystal silicon. More specifically, the present invention relates to an optical sensor comprising an amorphous silicon layer formed to bring into contact with a channel forming region of a bottom gate-type polycrystal silicon thin film transistor.
2. Description of the Related Art
Optical sensors are commonly used as linear image sensors or area image sensors that convert images produced by facsimiles, copiers, video cameras, digital still cameras, and the like into electrical signals. As materials for optical sensors include single crystal silicon, or an amorphous silicon layer is employed. However, except extraordinary cases, since images produced in a wavelength range of visible light are converted into electrical signals in most cases, the amorphous silicon layer with a greater light absorption coefficient for visible light is commonly used.
Optical sensors using amorphous silicon are divided into two major types: 1) a resistance-type and 2) a diode-type. In the resistance type, a greater current can be obtained due to amplification action as a transistor. However, since it produces carriers in large quantities by amplification, annihilation or collection of the amplified carriers are virtually impossible even after the light is interrupted, which results in a slower light response rate and narrower dynamic range controlled by the intensity of light. In the diode type, there is a feature that a depletion layer spreads widely in the amorphous silicon, thereby allowing the photo carriers produced upon the incident of light to be easily collected, and a faster light response rate due to the lack of amplification action and a wider dynamic range controlled by the intensity of light are obtained. However, because the current is small in the diode-type, a capacitor is required for retaining electric charges.
A switch to output signals detected by an optical sensor as an output signal with time-division has a bare IC-type that utilizes a field effect transistor of a single crystal semiconductor (mainly, silicon semiconductor) as an analog switch. Analog switches include TFT-types using a thin film transistor that employs amorphous silicon or polycrystal silicon for a channel forming region.
The IC types have a faster switching rate and a greater performance reliability but require as many analog switches as optical sensors as the bare IC chip, resulting in high costs switch applications. At the same time, since both a thin film substrate used to form a light absorption layer (optical sensor portion) such as amorphous silicon and the bare IC chip are required, the area thereof becomes wider, thereby being an obstacle of a size reduction. The TFT-types utilize thin films for forming the switches to allow both the optical absorption layer such as amorphous silicon and a TFT for use as a switch to be formed on the same substrate, thereby being capable of easily reducing the area and downsizing, and drastically cutting the costs compared to the IC-types. Among the TFT-types, thin film transistors using amorphous silicon for the channel forming region (amorphous silicon TFTs) utilize the amorphous silicon TFTs also for forming the switch element when, for example, the optical sensor portion is formed of amorphous silicon, resulting in lower cost than when utilizing polycrystal silicon TFTs due to sharing of the fabrication process. However, a faster switching rate is impossible due to the mobility of the amorphous silicon as small as 1 cm2/Vsec. For that reason, amorphous silicon is not applicable for an area sensor where area elements are increased in number, and for a linear sensor capable of dealing with high-speed.
Among the TFT-types, thin film transistors using the polycrystal silicon for forming the channel forming region (polycrystal silicon TFTs) require the formation of polycrystal silicon in addition to the formation of the light absorption layer such as amorphous silicon, resulting in more fabrication processes than in the case where the amorphous silicon TFTs are used. However, since the mobility of the polycrystal silicon is as great as 10 to 200 cm2/Vsec, a faster switching rate is possible. For that reason, an image sensor comprising the optical sensor element formed by the amorphous silicon and the switch element formed by the polycrystal silicon TFT is effective.
Most image sensors comprising the optical sensor element formed by amorphous silicon and the switch element formed by polycrystal silicon combine a diode type optical sensor utilizing the amorphous silicon and polycrystal silicon TFT for use as separate devices. The reason is that the use of the resistance-type optical sensor results in a reduced response rate, thereby making full use of the high-speed switching capabilities of the polycrystal silicon TFT impossible.
In most cases, either a 1) p-i-n diode or 2) Schottky diode is used for forming the diode-type amorphous silicon optical sensor element. The p-i-n diode forms a triple electro conductive layer of the p, i, and n types, where a depletion layer extends in the i-type amorphous silicon region, to thereby allow electrons to be transmitted into the n-type region and holes into the p-type region, with almost no recombination of the photo carriers produced therein.
The p-i-n diode-type structurally requires either the p-type or the n-type , utilizing silicon carbide (SiC), microcrystal silicon (xcexcc-Si), silicon nitride (SiN), and the like. The p-type and n-type layers require binary to quadrinary reaction gases, making the entire fabrication process complicated.
The Schottky diode-type forms a Schottky barrier by putting the amorphous silicon in contact with non-ohmic contact type conductive materials at the position thereof to use the resulting depletion layer formed in the Schottky barrier. The Schottky barrier is formed by simply forming a conductive film, which is much easier than the p-i-n diode-type. However, the depletion layer is formed in a narrower area compared to the p-i-n diode-type, making full collection of produced photo carriers difficult. A thinner amorphous silicon layer is required to collect all the photo carriers produced but has to tolerate smaller photo carrier quantities produced due to inferior optical absorption performance, resulting in lower photosensitivity for an optical sensor. A thicker amorphous silicon layer to increase optical absorption performance prevents the depletion layer from extending in the entire amorphous silicon and generates a resistance portion inside. That makes impossible to collect the produced photo carriers to recombine the same.
Either the p-i-n diode-type or the Schottky diode-type delivers a greater light absorption coefficient in shorter wavelengths of 450 nm and below when absorbing visible light to produce photo carriers, which causes the light to be absorbed before it reaches the depletion layer of the diode. This triggers recombination of the photo carriers produced by shorter wavelengths before reaching the depletion layer, resulting in no electrical signal outputs, which indicates weak sensitivity of blue color of an optical sensor.
In either of the Schottky diode-type or the p-i-n diode-type, the optical sensor element formed of the amorphous silicon and the polycrystal silicon TFT portion are formed in different locations. Therefore, upon the fabrication of the area sensor, the sensor portion and the TFT portion are formed in a single element. As a result, the region area of the light absorption element such as amorphous silicon and the like, which actually absorbs light, is reduced, making large-volume optoelectrical signal receptions difficult.
The present invention has been made in view of the above, and the present invention provides an entirely novel structure with a higher level of photosensitivity and operation rate by skillfully combining excellent absorption performance of amorphous silicon and high mobility of polycrystal silicon.
In other words, in an optical sensor according to the present invention, a photodetector portion using an amorphous silicon layer is arranged in contact with the upper portion of a polycrystal silicon TFT, a depletion layer at the source or drain portion of the polycrystal silicon TFT is extended to the interior of the amorphous silicon layer, and photocarriers produced by absorption by the amorphous silicon layer are possible to be swiftly transmitted to polycrystal silicon.
An object of the present invention is to provide an entirely novel optical sensor by combining excellent properties of the two materials, i.e., photosensitivity for visible radiation of amorphous silicon and high mobility of polycrystal silicon.
In addition, the light in short wavelength region easily realized a structure in which the photo carriers are produced not in the amorphous silicon layer but directly in a channel forming region of the polycrystal silicon. Therefore, the present invention has another object to provide a highly sensitive optical sensor to cover all of the visible radiation ranges of blue, green, and red.
Detailed description is made of means to achieve the above objects. Adjusting a gate voltage, included in the structure of a polycrystal silicon TFT, allows variations in field effect mobility especially in a channel forming region. For example, in a case of NMOS, the mobility becomes greater and the switch enters the ON state when positively charging the gate voltage. In reverse, the mobility becomes lower and the switch enters the OFF state when lowering or negatively charging the gate voltage.
This also applies to a case of PMOS by reversing positive and negative of the gate voltage. Forming an amorphous silicon layer in contact with the channel forming region allows photo carriers produced therein to be transmitted into the polycrystal silicon TFT when the switch is in the ON state.
To flow a current between a source and, drain requires application of a drain voltage in addition to the gate voltage. When the drain voltage is applied, a depletion layer is formed between the drain and channel. The state in which the depletion layer extends into the source is called xe2x80x98punchthroughxe2x80x99, whereby the current keeps on passing independently of the gate voltage, thereby requiring the drain voltage to be applied to an extent that the punchthrough is not formed.
The amorphous silicon layer is formed in contact with the drain and channel forming region. The depletion layer is formed between the drain and channel forming region. Since the amorphous silicon is in contact with the drain and channel formation region, the depletion layer can be formed from the drain through the inside of the amorphous silicon layer. The amorphous silicon layer allows the depletion layer extending from the drain to be formed therein when the drain voltage is applied, to transmit photocarriers produced in the amorphous silicon layer to the channel forming region immediately after the production by the depletion layer.
Processing amorphous silicon with the solid phase growth method to make it polycrystalline forms the polycrystal silicon. The amorphous silicon layer as a photodetector should be formed after the solid phase growth. If the amorphous silicon is formed before the solid phase growth, it is crystallized during the solid phase growth of the polysilicon film, or although crystallization may be avoided, hydrogen flows out of the amorphous silicon in large quantities, resulting in the amorphous silicon formed with dangling bonds in large quantities. This leads to traps of photo carriers by the dangling bonds, making photodetecting impossible.
A top gate type TFT comprises polycrystal silicon, a gate insulating film, and a gate electrode formed on a substrate in bottom-to-top sequence from the substrate according to the order. In this case, since the channel region is formed below the gate, it is impossible to make the amorphous silicon film in contact with the channel region. In order to have the amorphous silicon layer contact with a drain region, since the amorphous silicon layer can not be formed on the upper portion of the TFT, it needs to be formed between the TFT and the substrate. However, amorphous silicon needs to be formed prior to formation of polycrystal silicon, making the formation of amorphous silicon layer capable of photodetecting virtually impossible, as described above.
To solve the problem, the present invention employs a bottom gate type TFT comprising a gate electrode, a gate insulating film, and polycrystal silicon formed on the substrate in bottom-to-top sequence from the substrate according to the order to form the amorphous silicon layer on the polycrystal silicon. This structure is free of the problem described above as the amorphous silicon layer is formed after formation of the polycrystal silicon TFT. In the polycrystal silicon layer, the channel forming region is formed over the gate electrode, sandwiched between the source and drain regions. Forming amorphous silicon on top of the polycrystal silicon layer thereby allows the amorphous silicon layer to easily come in contact with the channel formation region and the drain region of the polycrystal TFT.
Forming the amorphous silicon layer in contact with both the drain and channel forming regions allows the depletion layer formed between the drain and channel to extend further to the inside of the amorphous silicon layer. Photo carriers produced in the depletion layer are smoothly collected into the channel forming region.
In a case of the bottom gate type TFT, the amorphous silicon layer formed in contact with the TFT is not in contact with the channel itself in actuality. The channel is formed on the gate insulating film and polycrystal silicon interface. The TFT, however, comprises the polycrystal silicon layer as thin as 100 to 1500 xc3x85 approximately, resulting in the depletion layer formed in the entire region of the polycrystal silicon layer in a vector perpendicular to the polycrystal silicon layer from the gate electrode when applying a gate voltage. Photo carriers produced in the channel and amorphous silicon layer in contact with the polycrystal silicon layer opposite the channel reach the channel forming region (i.e. the polycrystal silicon layer in which the channel is formed, but is not channel itself) and then reach the channel along the resulting depletion layer.
In addition, by forming a transparent conductive film on the amorphous silicon layer to apply a voltage between the transparent conductive film and the source region, effective collection of photo carriers is obtained. Coupling the transparent conductive film which is transparent to visible radiation to the drain allows a simple structure in which a drain voltage can be applied also between the transparent conductive film and source. Further, the contact between the transparent conductive film and the amorphous silicon becomes a Schottky junction, thereby forming a depletion layer by the resulting Schottky barrier. The depletion layer extending from the drain region to the inside of the amorphous silicon layer, an electric field induced between the transparent conductive film and the source, and the depletion layer by the Schottky barrier cause collection of photo carriers in the channel forming region. The channel substantially acts like conductor when a gate voltage is applied and the TFT is in the ON state, resulting in electrical fields produced not only between the transparent conductive film and source but also between the transparent conductive film and channel, to collect photo carriers in the channel forming region effectively.
When forming no transparent conductive film, the amorphous silicon layer is covered with an insulating film transparent to visible radiation in order to avoid degradation of the amorphous silicon layer. In this case, photo carriers produced by the light absorbed in the depletion layer extending from the drain region to the inside of the amorphous silicon layer and dispersed photo carriers produced by the light absorbed outside the depletion layer are collected in the channel forming region.
The amorphous silicon layer has dark resistivity of 1xc3x971010 xcexa9cm and more delivering relatively great resistivity to the dark resistivity of polycrystal silicon of 106 xcexa9cm (by triple digits and more). This prevents a leak current from passing via the amorphous silicon layer when the polycrystal silicon TFT is in the OFF state.
There are problems associated with amorphous silicon layer light absorption of visible radiation having shorter wavelengths (450 nm and less). The amorphous silicon layer delivers a greater absorption coefficient for light of shorter wavelengths, resulting in absorption of almost all incoming light by the amorphous silicon layer before the light reaches the depletion layer extending from the drain. Though the absorbed light subsequently produces photo carriers, not all of the photo carriers reach the depletion layer. Many of the carriers recombine before reaching the depletion layer, only to lower photosensitivity.
Having as much light of shorter wavelengths as possible absorbed either in the channel or near the channel forming region of the polycrystal silicon TFT allows produced photo carriers efficiently to be transmitted into the channel. A structure in which the amorphous silicon layer is not formed in a part of the channel forming region of the polycrystal silicon TFT allows efficient absorption of light having either longer or shorter wavelengths. The amorphous silicon layer is formed in contact with a part of the drain region and a part of the channel forming region, allowing a depletion layer formed in the drain to extend further to the inside of the amorphous silicon layer, and photo carriers produced therein to be collected into the channel. At the same time, light is directly irradiated into the part of channel forming region out of contact with the amorphous silicon layer, (via an etch stopper film with a wide energy gap in actuality).
The polycrystal silicon has a thickness of 100 to 1500 xc3x85 approximately for a film to form a thin film transistor and also for reducing OFF-state currents. The layer is thick enough to absorb light of shorter wavelengths, and the light of shorter wavelengths absorbed in the channel forming region is instantaneously collected into the source through the channel.
Implementation of an optical sensor with a high level of photosensitivity for all of the light of the visible radiation regions from shorter to longer wavelengths of 300-800 nm approximately is applicable by disposing the channel forming region out of contact with the amorphous silicon layer.